Freshly mined gypsum is known as landplaster. It includes calcium sulfate dihydrate, and is also known plaster or terra alba. In many deposits, landplaster includes at least 50% by weight calcium sulfate dihydrate. Synthetic gypsum may also be known as Landplaster. Synthetic gypsum is a sulfate material that results from the flue gas desulfurization (FGD) or “scrubbing” process at coal-fired power plants. Calcining the calcium sulfate dihydrate at temperatures above 150° C. drives off part of the water of hydration, forming calcium sulfate hemihydrate, also known as calcined gypsum, stucco, Plaster of Paris, calcium sulfate hemihydrate or calcium sulfate half-hydrate. When added to water, calcium sulfate hemihydrate crystallizes with the water to reform the dihydrate. As the reaction proceeds, an interwoven matrix of calcium sulfate dihydrate crystals forms and becomes hard. The reaction is represented by the formula: CaSO4.½O+3/2H2O→CaSO4.2H2O.
Many substances are known to accelerate the hydration rate of calcium sulfate hemihydrate hydration. Set time is determined by one of two mechanisms. There is an initial delay or induction period while a few “starter” crystals are formed. After this induction period, the rate of reaction increases to a standard rate. Most set accelerators increase the hydration rate. For example, most sulfate compounds act as set accelerators according to LeChatlier's principle. Few substances are known to reduce the induction period. The most well-known of these is calcium sulfate dihydrate.
Mined, unground landplaster is ineffective as a set accelerator for hydration reactions. Grinding calcium sulfate dihydrate exposes fresh nucleation sites that speed formation of the dihydrate gypsum. Exposure to humidity deactivates the nucleation sites in as little as a few hours. To preserve the active surfaces, it is known to treat ground calcium sulfate dihydrate with a carbohydrate, such as sugar, to prevent oxidation. When added to a slurry of calcined gypsum and water, the carbohydrate quickly dissolves and exposes the active crystal sites. The active crystal sites act as “seed” crystals that facilitate formation of calcium sulfate dihydrate crystals in an interlocking matrix.
Examples of coated calcium sulfate are heat resistant accelerator (HRA) and climate stabilized accelerator (CSA), described in U.S. Pat. Nos. 2,078,199 and 3,573,947, respectively. U.S. Pat. Nos. 2,078,199 and 3,813,312 disclose when sugar or soluble dextrin are ball-milled together with the calcium sulfate dihydrate, the material functions to prevent dehydration of the accelerator, even when subjected to various atmospheric conditions on storage. Dextrins are a group of low-molecular-weight carbohydrates produced by the hydrolysis of starch or glycogen. Dextrins are mixtures of polymers of D-glucose units linked by α-(1→4) or α-(1→6) glycosidic bonds. Additionally, U.S. Pat. No. 3,870,538 discloses starch may be finely ground together with calcium sulfate dihydrate to serve as a preservative of the accelerator. U.S. Pat. No. 4,019,920 describes a gypsum set accelerator produced by compression milling a mixture of ground gypsum rock and starch, and the addition thereof to an aqueous slurry of settable calcined gypsum such as in the forming of paper-covered gypsum wallboard. Corn starch is preferably used in this reference. U.S. Pat. No. 6,221,151 describes a composition for accelerating the setting reaction of an aqueous calcined gypsum slurry including a vibratory ball milled mixture of calcium sulfate dihydrate and a surfactant. U.S. Pat. No. 6,221,151 discloses suitable surfactants for use in its invention include, but are not limited to, anionic, cationic, nonionic surfactants, and mixtures thereof. Suitable anionic surfactants include, but are not limited to, linear alkylate sulfonates such as sodium alkylbenzene sulfonates and sodium lauryl sulfates. Suitable cationic surfactants include, but are not limited to, starches, such as corn starch, acid-modified corn starch, grain starch, pearl starch (i.e., an unmodified raw starch), wheat starch, and acid-modified wheat starch. Suitable nonionic surfactants include, but are not limited to, ethoxylated nonylphenols. Generally, suitable starches include those which, when dispersed with a wet, set gypsum slurry core, will migrate to the core-to-paper cover sheet interface during drying and provide a strong bond at the interface between the core and paper cover sheet.
While these additives for accelerators have benefits, these materials, particularly dextrin, sugar, e.g., corn sugar, dextrose, glucose, sucrose, or other sugar are very expensive.
Another known accelerator is described in U.S. Pat. No. 6,409,825. This accelerator, termed a wet accelerator, includes ground calcium sulfate dihydrate in water with at least one additive selected from an organic phosphonic compound, a phosphate-containing compound or mixtures thereof. As with CSA and HRA, the gypsum particles act to accelerate the initial crystallization during the hydration reactions. This particular wet accelerator exhibits substantial longevity and maintains its effectiveness over time such that the wet gypsum accelerator can be made, stored, and even transported weeks or months after its manufacture. The wet gypsum accelerator is used in amounts ranging from about 5 to about 80 pounds per thousand square feet (24.3 to 390 g/m2) of board product. Another wet accelerator is described in WO 2006/115496, which is formed by grinding calcium sulfate dihydrate, water and a phosphate additive, e.g., pyrophosphate, tripolyphosphate, or trimetaphosphate.
Another known accelerator is described in U.S. Pat. No. 4,298,394. This accelerator comprises a milled composition of gypsum and sodium alkylbenzene sulphonate (up to 20 wt. % of the gypsum) added to an aqueous slurry of stucco to prepare a set gypsum. U.S. Pat. No. 4,059,456 describes a composition made up of finely ground calcium sulphate dihydrate and lignosulphonate, a waste product derived from the sulphite pulping of wood. Canadian Pat. No. 999,310 describes a fine particulate intimate admixture of gypsum and a long chain fatty carboxylic acid containing at least 12 carbon atoms, or a salt thereof.
WO 1997/021637 describes naphthalene sulfonate as a grinding aid but it was used in a Portland cement composition, not a gypsum composition.
Pregelatinized starch has been used in making gypsum wallboard. In this regard, U.S. Pat. No. 8,262,820 describes a gypsum-containing slurry including stucco and an aqueous pre-dispersion of a pregelatinized starch, as well as a method of making gypsum wallboard by providing a pre-dispersion of the pregelatinized starch in water, and adding the pregelatinized starch pre-dispersion to the gypsum-containing slurry. The pregelatinized starch can have a particle size distribution within the following range: d (0.1)=about 20μ-35μ, d (0.5)=about 60μ-110μ, d (0.9)=about 100μ-220μ. It discloses pregelatinized corn starch ground to specific particle size ranges to be pre-dispersed in water and added to a stucco-containing slurry, and discloses an accelerator made from dry grinding of gypsum with additives, such as sugar, dextrose, boric acid, and starch added to the stucco-containing slurry. However, it does not disclose co-grinding of pregelatinized starch with gypsum to make an accelerator.
It will be appreciated that this background description has been created by the inventors to aid the reader and is not to be taken as an indication that any of the indicated problems were themselves appreciated in the art. While the described principles can, in some aspects and embodiments, alleviate the problems inherent in other systems, it will be appreciated that the scope of the protected innovation is defined by the attached claims and not by the ability of any disclosed feature to solve any specific problem noted herein. Thus, there is a continuing need for new and improved set gypsum-containing products and compositions used in preparing the products, particularly set accelerators, as well as methods for producing them, that solve, avoid, or minimize a problem noted above, and/or improves properties of the products.